WASHINGTON—At least two groups of China-linked hackers have spent months using a previously undisclosed vulnerability in American virtual private networking devices to spy on the U.S. defense industry, researchers and the devices’ manufacturer said Tuesday. Utah-based IT company Ivanti said in a statement the hackers took advantage of the flaw in its Pulse Connect Secure suite to break into the systems of “a very limited number of customers.” Ivanti said that while mitigations were in place, a fix for the issue would be unavailable until early May. Ivanti provided no detail about who might be responsible for the espionage campaign but, in a report timed to Ivanti’s announcement, cybersecurity company FireEye Inc said it suspected that at least one of the hacking groups operates on behalf of the Chinese government. “The other one we suspect is aligned with China-based initiatives and collections,” said Charles Carmakal, a senior vice president of Mandiant, …